1. Field of the Invention
The present invention relates to a developing apparatus for use in an electrophotographic image forming apparatus such as copying machine, laser beam printer, or faximile, and more particularily to the developing appratus for providing a photoreceptor with toner particles to develop an electrostatic latent image on the photoreceptor into a visible powder image.
2. Description of the Prior Art
One known developing apparatus comprises a developer roller disposed adjacent to an electorostatic latent image bearing member or photoreceptor, which roller consists of a stationary magnetic member having a plurality of magnetic poles on its outer periphery, only one of said magnetic poles confronting to the photoreceptor, and a rotatably supported sleeve enclosing the magnetic member therein. Developer material of two-component type is retained on an outer periphery of the sleeve by the effect of a magnetic force generated by the magnetic member. The developer material on the sleeve is, during a rotation of the sleeve, transported so that the developer material retained by the magnetic pole facing to the photoreceptor is brought into contact with the photoreceptor to develop an electorostatic latent image into a visible powder image.
The prior art developing apparatus has several drawbacks. Namely, since the developer material retained only by the single magnetic pole confronting to the photoreceptor is brought into contact with the photoreceptor, the width of an area of contact of the photoreceptor of the sleeve with respect to the direction of rotation of the sleeve is rather small and, therefore, the density of the developed images varies with change in amount of electric charge of toner which may take place as a result of change in environment. For example, the image density reduces as humidity decreases.
Further, if in order to secure a proper image density the developer material is quite densely retained at the region where the developer roller confronts the photoreceptor, part of toner particle deposited upon the photoreceptor is scratched off by magnetic brush brisles then moving away from the photoreceptor, leaving a mark of scratching the toner deposit on the photoreceptor which eventually result in formation of the developed image having white lines corresponding to the mark of scratches.
Moreover, in a developing apparatus having a regulating plate facing to the periphery of the sleeve with a small gap defined therebetween in order to regulate the amount of the develper material to be transported to the region where the sleeve confronts to the photoreceptor, if foreign matter of a large size such as scraps or lumps of concenrated toner is caught in the gap between the regulating plate and the sleeve, the develper material carried by the sleeve is blocked by the foreign matter, eventually resulting in formation of a defective image bearing undesirable white line.